The installation and performance characteristics of a 20 W sodium beacon laser system for the 3 m Shane telescope at the Lick Observatory are presented

Erbert, G.

Friedman, H.

Kuklo, T.

UNT (University of North Texas) Digital Library

Thisisa preprint of a paper intended for publication in a journal orproceedings. Since changes may be made before publication, this preprint is made available with the understanding that it will not be cited or reproduced without the permission of the author. UCRL-JC-119380 PREPRINT Sodium Beacon Laser System for the Lick Observatory H. Friedman, G. Erbert, T. Kuklo, T. Salmon, D. Smauley, G. Thompson, J. Malik, N. Won& K. Kanz, and K. Neeb This paper was prepared for submittal to the SPIE 1995 International Symposium on Optical Science, Engineering, and Instrumentation San Diego, CA July 6,1995 July 9-14,1995 DISCLAIMER This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process d i s c l d ,  or represents that its use would not infringe privately owned rights. Referenae herein to any specific commeraal product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or the University of California, and shall not be used for advertising orproductendorsementpurposes. DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. Sodium Beacon Laser System for the Lick Observatory Herbert Friedman, G a y h  J3bert, Thomas KuMo, Thaddeus Salmon, David Smauley, Gary Thompson, Jody Malik, Nan Wong, Keith Kanz, and Kurt Neeb Lawrence Livemore National Laboratory P. 0. Box 808, L-464 Livemore, CA 94550 ABSTRACT . The installation and performance characteristia of a 20 W sodium beacon laser system for the 3 m Shane telescope at the LiCkObservatayare~ted. 1. INTRODUCTION ?he design and performance prediaions of a 20 W laser system for the 3 m S h e  telescope at the Lick Observatory were presented m apviouspaper.l In Juneof 1B5. the laser was propagated from the Shane domeas shown in Figure 1 and the integration, alignment and Commissioning process is now underway. In this paper, the hardware is described, installation details dkused, and the laset performance pmented. Figure 1. Propagation of the sodium guide star laser from the 3 m Shane dome at the Lick Observatory. 2. GENERAL LAYOUT A key feature of this laser system is the set of dye laser ampWiers which can be pumped by incoherent light delivered by optical f i b .  Tbe pump laseas and wave form generator can therefore be located remotely from the telescope in order to fhcilitate the removal of waste heat from the dome area as shown in Figure 2. The dye laser amplifkn are then located directly on the telescope. bore sighted to the axis of the telescope and lodced down securely thereby eliminating the need for complex beam directors. There is an issue as to whether the laser beam should be transported saws the primary mirmr and propagated from behind the secondary or simply launched from the side. In numerous calculations and simulations, it has been found that the side or off-axis propagation does introduce some &gradation in fitting error of the wave front sensor, but that the error is indeed small, of the order of 0.01 Strehl units, and is not worth the considerable effort to accomplish hue coaxial propagation. SHANE 3 MFTER DIAGNOSTICS DOME AREA DARK ROOM Figure 2. Design for telescope mounted, off axis dye laser amplifiers and remotely located support equipment. A photograph of the laser installation on the Shane 3 m telescope is shown in Figure 3. The optics table at the base of the telescope contains the dye laser preamplifier and power amplifier while the table at the mid level contains beam control, wave front and power monitoring diagnostics. The launch telescope is the tapered structure at the top which culminates in a 30 cm refractive optic. Optical fibers containing the pump light and dye laser signal are contained in a small, separate umbilical tube while the dye lines are housed in the larger tube. Signal, control and video links are lashed alongside with the entire bundle routed in a cable wrap similar to other cables on the telescope. r V Figure 3. Installation of the 20 W dye laser system on the 3 m telescop. Prior to installation on the telescope, the entire laser system was assembled and integrated on a (vertical) optics bench as shown in Figure 4. The laser table containing the dye amplifiers was mounted on a rotation table so that its mechanical the diagnostics tables was simulated by fold mirrors but the connections between the diagnostic table and launch telescope was full scale. All fiber connections between the pump lasers and wave form generator were full length (60 m) in order to simulate actual performance. a C stability could be verified as a function of (simulated) telescope motions. The six meter distance between the laser and r Dye lines . . .  . .  . ~ ' . .  ..,. . . .,..*... "';*;24;' .. ::..ab .., .. ..,' . . .-.< 3o em. 19 outputd : r  ., ." =-- Laser pump fibors Figure 4. Integration of the guide star laser system prior to installation on the telescope. The auxiliary support equipment consist of a set of three frequency doubled YAG lasers, wave form generator, two racks of electronics modules, the dye pump system with chiller, the safetyhterlock control computer (PLC), and a Macintosh computer work station with LabView software. The wave form generator consists of a Dye Master Oscillator, phase modulator and wavelength control diagnostics and is described in Reference 1. The entire support facility is housed in an air conditioned room measuring 8 feet by 20 feet. Advanced designs would rack mount the YAG lasers, integrate the YAG laser cooling, dye pumps and dye cooling into the observatory facilities, and incorporate the work station into the centralized control room. In this case, the support equipment would fit into a space measuring 6 feet by 10 feet. 3. LASER TABLE An optical layout of the laser table is shown in Figure 5 .  A 5 p single mode fiber from the wave form generator appears in the lower right of the figure followed by a microscope objective to collimate the beam. The power level in the fiber is several milliwatts corresponding to a specific peak power of about 10 MW/cm2 for these 150 ns. 10 kHz pulses. Beyond this power level, nonlinear effect.. limit the power handling capability of these fibers. It is to be noted that these nonlinear limits are increased by the modulation format and that narrow bandwidth signals have lower limits. The limited power transmission of single mode fibers is the penalty for the remote location of the wave form generator and a preamplifier on the laser table is needed to restore the dye laser power to the level of the Dye Master Oscillator (DMO) which is the source of the wave form generator. In fact die dye cell of the preamplifier is identically the same as for the DMO only minus the cavity optics and control system. 2' x 4' optics table A7 power amplifier .5mnlQ.5mnx2ofml Figure 5. Optical layout of &be dye laser table. The output of the preamplifier at the several hundred milliwatt level is filtered by the (square) pinhole slit which is then resized and (relay) imaged to the center of the power ampWier. Pointing and centering is also accomplished by this Dye Relay Telescope. The power amplifier has a spacing of 0.5 mm and length of 2 cm with a flow of 1.5 gpm corresponding to a dye flow rate of Id cm/s or two dye intercbanges per pulse. The input signal to the power amplifier is of the order of 50 mW and the output signal is 22.5 W with a pump power of 150 W ( as measured at the YAG laser) as shown in Figure 6. These figures correspond to a overall efficiency of about 15% including a fiber loss of about 25% for the 60 m length and Fresnel loss at the uncoated fiber ends, and an additional 10% loss in the fiber relay telescope which images the fiber ends into the dye cell. Without these loses, the intrinsic dye conversion efficiency corresponds to 22%. LICK A 1  PERFORMANCE . . . . . . . . . . . . . . . . . . . . . . . . .  . . . . . . . . . . . . . . . . . . . . . . . . .  . .  . . . . .  . . . . . . . . .  . . . . . . .  . . . . .  . . . . . .  . .  . . . . . . . . . . . . . . . . . . . . . .  . . . .  . . . . . .  . . . . . . . . . .  / / ;  . . . . . . .  . . . . . .  0 20 40 60 80 100 120 140 160 1 8 0  200 WMP P M R  (WATTS) Figure 6. Performance characteristics of the dye laser power amplifier. Scaling this technology to higher power levels such as would be required for visible wavelength astronomy, would require a second power ampWier. There is already sufficient mom on the dye laser table for a second amplifier as is evideat from the photograph of Figure 7. This second power ampMiiea would be identical to the first but with a larger gap of 1 mm. The efficiency of this second amplifier should be higher than the first. The gain of the first amplifiex is approximately 20 W/50 mW = 40 which put this device in the small signal, low efficiency region of opetation. The second amplifier would operate in the saturated, high efficiency region with gains of 5-10 and should have an efficiency >25%. Under these conditions the number of YAG pump lasers required to reach the 100-200 W level is in the range of 5-10. 4. DIAGNOSTICS AND ALIGNMENT TABLE An optical layout of the Diagnostics & Alignment @&A) table is shown in Figure 8. The first optic on the table is a commercial, high speed tilt stage (Physic Instrumente) which removes all tilts of the laser beam including those induced by the laser, optics, atmosphere, etc. These tilts are sensed by the wave front sensor in the Adaptive Optics system which views the mesospheric guide star and are transmitted to the tilt stage forming a closed loop control system. After a turning mirror, the beam is passed through a radar controlled safety shutter which is part of the aircraft avoidance system. In the closed mode, the beam is dumped into a powex meter. ' retro mirror I -r I output shutter I Pointing I I wavefront sensor I I (Shack-Hartmann) Figure 8. Optical layout of the D&A table After the safety shutter but before the primary lens of the launch telescope, a beam splitter samples the laser and inserts a portion of the beam into a P&C diagnostic package. The cross coupled output of this diagnostic package controls the "optics-couple'' formed by the turning mirror and relay lens on the laser table to maintain pointing and centering to a fmed position. usually determined by alignment of b e  guide star on the science objects in the Adaptive Optic system. The rest of the sampled beam is retro reflected into the on-board H m a n n  wave front sensor which is calibrated by the wave front reference. This wave front reference is generated by a single mode HeNe laser operating at 594 nm and located near the pump lasers. The output of this laser is transmitted to the D&A table via another single mode fiber where it is expanded and collimated in an Invar mounted optics system. Since the laser beam is not collimated at this point, the beam will have several waves of divergence from the rely lens. This divergence is "subtracted" by comparing the forward going laser beam with a reuo reflection from generated by the flat surface of the 30 cm lens which forms the primary of  the launch telescope. In practice, the wave front sensor operates by first taking an exposure of the wave front reference, followed by a second exposure of the outgoing laser beam and finally, an exposure of the retro wave from the launch telescope. The three exposures are controlled by shutters in the optics system and require about 3 seconds for a ful l  sei. An example of the wave front leaving the dome as measured by these three exposures is shown in Figure 9. The rms wave front error is about 1/7 waves which will result in a Strehl ratio in the mesosphere of about 45% according t o  the Mareshal approximation, exp(-02) where (T = (1/7)2rc. Figure 7. Photograph of the dye laser table with the cover removed. The final lens on the laser table is used to relay the image of the center of the power amplifier to the exit of the launch telescope thereby eliminating diffraction effects which would otherwise increase the spot diameter in the mesosphere. Alternatively the size of the output optic could be increased to capture the diffraction spreading but only at a significant cost increase. The minimum separation distance between the power amplifier and the relay lens is about 40 an dictated by diffraction effects with increase the axial intensity in the near field. The relay lens has motion capability in the plane transverse to its axis as part of the Pointing & Centering (P&C) loop. + Ve$ rms =om wave$ sagy = -227 waves n n ~  = 1.37 W V ~ $  sagy = 2.67 W V ~ S  Shading clipped Shading clipped outgoing wave rms = 0.14 waves sagy = -0.14 waves Shadw dipped Figure 9. Wave front of the laser beam leaving the launch telescope as measured by the on-board Hartmann sensor. The purpose of all these diagnostics is to ensure the quality and stability of the laser beam before i t  leaves the dome. This philosophy is in contrast to simpler schemes which use the spot in the mesosphere as the primary diagnostic. I t  has been our experience in both laser guide star experiments as well as complex integration demonstrations involving laser systems and adaptive optics, that it is essential to have a full suite of diagnostics on-board the laser system in order to rapidly acquire the laser beam and reduce down time when difficulties do occur. This is especially important in the astronomical application where telescope time is quite valuable. A photograph of the D&A table is shown in Figure 10 where the main beam path has been indicated by white streak. Figure 10. Photograph of the Diagnostics and Alignment table with the main beam path indicated in white. 5. SUMMARY A 20 W sodium guide star laser has been installed on the 3 m Shane telescope at the Lick Observatory. The use of dye amplifiers and optical fibers have allowed the frequency doubled YAG pump lasers and tile dye laser wave form generator to be remotely located leaving the dye amplifiers to be strapped directly to the side of the telescope. With the measured wave front quality of 1/7 wave (rms) and the pulse duration of 100 ns FWHM), the peak power intensity in the -- 60 cm spot diameter will be a few watts per square centimeter, which is below the saturation flux of = 5 W/cm2. The laser has been operated at the 20 W level for several hours at time with amplitude stability of +58 with fine adjustments on a 10-20 minute basis. Closed loop control systems developed at LLNL could be applied to the wave form generator which would extend the adjustment period to the level of an hour or more. Unfortunately, the combination of poor weather and limited access to the telescope have delayed the final bore sight alignment of the laser launch telescope to the 3 m telescope. At this point. we have not been able to acquire the laser guide star in the acquisition camera of the 3 m telescope in order to measure iu diameter or return signal. We expect to complete this task in the coming months. An adaptive optics package consisting of a Hartmann wave front sensor, continuous face sheet deformable mirror. tilt sensor and mirror stage. and the Lick Infrared Camera (LIRC III2 has been tested on the 3 m telescope using natural guide stars. The system has produced diffraction limited images at 2.2 p aiid has improved Uie seeing from the arcsecond lewl lo <1/4 arcsec. The laser system will be integrated with this adaptive optics package in the coming months for the first sodium-layer guide star. closed loop adaptive optics system. This work tvas performed under the auspices of the U.S. Department of' Enersy hy Lawrencc Livcrnwre National Laboratory under contract No. W-7405-ENG-48. J 6. REFERENCES 1. H. W. Friedman, et aL., SPIE Paper 2201-36 Adaptive Optics in Astronomy (1994)/352 2. S. Olivier et al., SPIE Paper 2534-03, Adaptive Optid Systems and Applications (1995) < 

Sodium beacon laser system for the Lick Observatory

https://digital.library.unt.edu/ark:/67531/metadc627627/m2/1/high_res_d/102370.pdf

Sodium beacon laser system for the Lick Observatory

Abstract

Similar works

Full text

Available Versions

UNT (University of North Texas) Digital Library